U.S. Pat. No. 5,124,716, the disclosure of which is hereby incorporated by reference herein, discloses a multi-orifice ink jet print head for ejecting ink drops onto a print medium, such as paper. The multi-orificed ink jet print head 25 is shown with associated elements in FIG. 1. An acoustic driver, such as a piezoelectric transducer 32, is coupled to a diaphragm 34 for ejecting ink drops from an ink chamber 12, through a nozzle orifice 18, and onto a print medium 19. The piezoelectric transducer 32 comprises first and second conductive electrodes separated by a layer of insulating piezoelectric material. A control signal provided by a signal source 56 is applied to the transducer and the diaphragm 34 is displaced according to the voltage of the control signal.
FIG. 2 shows a known unnormalized waveform of a control signal that may be provided by the signal source 56 for driving the piezoelectric transducer 32. The signal has a positive pulse of +Vo volts which lasts for about 5 .mu.s and then returns to 0 volts. The signal remains at 0 volts for a period of time T1. A negative pulse of -Vo volts, follows the period T1 and lasts for a second period T2 before returning to 0 volts. During the positive pulse, the piezoelectric transducer displaces the diaphragm away from the cavity interior, and ink from reservoir 14 is drawn into the cavity 12. In response to the negative pulse, the diaphragm is displaced for compressing the cavity and an ink drop is ejected from the orifice 18 onto the print medium 19.
When placing an image on the print medium, the print head 25 shuttles back and forth along the X-axis parallel to the plane of the print medium surface and the print medium advances along the Y-axis perpendicular to the X-axis while the jets of the print head eject drops onto the print medium. The quality of the resulting image depends upon the size and velocity of the drops produced by each jet of the array of jets of the print head. Drop size affects the color density of an image while velocity affects the placement of dots with respect to other dots in the image. Ideally, each jet of the print head performs similarly to the other jets of the print head and each print head is manufactured with optimum parameters for ejecting ink. However, because of limited controls during manufacturing, performance variations exist.
Many parameters affect the performance of ink jets. Temperature non-uniformities across a print head will produce variations in ink viscosity for the different jets of the print head. Drop production is affected by driver efficiency, which changes according to parameters such as thickness of the layer of piezoelectric material, stiffness of the diaphragm and the piezoelectric material, density and piezoelectric constant of the piezoelectric material and coupling coefficient between the electrodes and the piezoelectric material. Alignment of the acoustic driver with respect to the ink jet chamber and the coupling interface between the acoustic driver and the diaphragm of the ink chamber also affect drop production. Because of the limited control over these and other ink jet parameters, production lots experience variations in jet performance. By adjusting the waveform of the control signal applied to the acoustic driver, drop size and/or velocity may be altered and variations in jet performance may be partially compensated.
It is known from U.S. Pat. No. 5,124,716 to adjust the waveform of the control signal by changing the timing intervals, T1 and T2 of FIG. 2.
U.S. Pat. No. 5,212,497 which is assigned to the assignee of the present invention and the disclosure of which is hereby incorporated by reference herein, discloses a normalization technique wherein the drop ejection velocity of a jet is monitored by using a strobe imaging device to strobe ejected drops while adjusting the attenuation of the output signal provided by a signal source to produce the control signal applied to the jet's piezoelectric transducer. Referring to FIG. 3, changing the amplitude of the control signal V.sub.cntrl changes the amount by which the acoustic driver 32 displaces the diaphragm 34 of the ink jet and thus affects drop ejection velocity. The control signal received by the piezoelectric transducer is controlled by adjusting a potentiometer R.sub.POT, which contributes to the series resistance (R.sub.POT +R.sub.SA) of a divider network 36. After adjusting the potentiometer for an optimum ejection velocity, the series resistance is measured and data representative of the optimum series resistance is recorded. This recorded data is sent to a resistor trim production step where the series resistor R.sub.SA of the resistor divider network 36 which is in the series path between the drive signal source 56 and the acoustic driver 32 is trimmed according to the received data. To produce a normalized print head in which each jet is tuned for uniform performance, the strobe imaging/potentiometer adjustment and the subsequent series resistor trim steps are performed for each jet of the print head. As such, the resistor trim normalization technique requires a significant amount of time for performing the normalization steps for all of the jets of the multiple-jet-array print head. In addition, the divider network dissipates power when attenuating the control signal and therefore consumes extra energy when used to attenuate the control signal and affect jet performance.
These problems are solved in the method and apparatus of the present invention.